This invention relates generally to fabric drying apparatus and more particularly to control circuitry therefor. The control circuitry for the fabric drying apparatus includes the usual cycling and high limit thermostats as well as thermistor based oscillator circuitry for temperature sensing. The frequency of oscillation changes in response to changes in airflow temperature and is monitored by a microcontroller. The detection of oscillation frequencies outside a predetermined range, and therefore temperatures above a predetermined limit, indicate failure of either the cycling thermostat or a thermistor circuit component and will cause the microcontroller to effect deenergization of the fabric drying apparatus.
The prior art includes U.S. Pat. No. 4,195,500 issued on Apr. 1, 1980 to Tobita et al. This patent teaches an automatic washer having a weight sensor associated with the tub for sensing the weight of and therefore the quantity of water in the tub. If a predetermined rate of change of weight is detected in a predetermined time period during a fill the cycle will proceed. During drain, if a predetermined change in weight does not occur in a predetermined time, it is determined that the drain unit is faulty. If the vibration of the drive motor is not sensed through the weight sensor, it is determined that the motor is locked. Each of these determinations is made at predetermined times in a cycle of operations and the control will not allow further operation until the particular problem has been resolved. Three indicator lights will flash in wash, rinse or dehydration modes depending on cycle position if operation is normal. Abnormal operation is indicated by various combinations of ON and flashing indicator lights.
Proctor, U.S. Pat. No. 4,393,300 issued on July 12, 1983, is representative of temperature sensing thermistor circuits as used in microwave ovens for determining the end of the heating process.
The prior art has thus shown the sensing of the rate of change of tub weight in a washing machine for detecting malfunctions in filling, washing and dehydration. The vibrations relating to motor operation are also sensed through the same weight sensor. Thermistor temperature sensing circuitry has also been utilized in microwave oven design for determining the heating cycle end point. There has been, however, no known showing of thermistor based oscillator circuitry for sensing the temperature of airflow through fabric drying apparatus where the thermistor based circuitry provides temperature sensing inputs to a microcontroller and detects an oscillation frequency outside of a predetermined range to indicate failure of certain circuitry components. Detection of frequencies outside of the predetermined range causes the microcontroller to effect deenergization of the fabric drying apparatus and to effect the display of a coded message corresponding to faulty temperature sensing components.